A Supernova May Light Up the Sky in Six Years

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A
dim binary star is behaving exactly as expected if it is about to explode as a
“red
nova“. If that happens, in 2022 or so it could shine as brightly as the
North Star. Dozens of ordinary novae – the temporary flare-ups of white
dwarf stars stealing gas from their companion star – explode in our
galaxy every year. These novae turn blue.

In
recent years, however, astronomers have discovered a rare type of nova that
turns red instead. At peak brightness, many red novae rival the most luminous
stars in the galaxy. A red nova in 2008 gave us a clue as to why these
explosions happen: observations made before the blast revealed that the nova
was the result of two stars orbiting each other merging into one.

The
two stars were in a so-called contact binary, orbiting so closely that they
touched. If Earth circled a contact binary, our suns would look like a fiery
peanut. Despite their exotic appearance, contact binaries are common, with
nearly 40,000 known in our galaxy. Now, new observations show that one, named
KIC 9832227, could be about to explode as a red nova.

Boom
star

“My
colleagues like to call it the ‘Boom Star’,” says Larry Molnar of Calvin College
in Grand Rapids, Michigan.

The
binary is roughly 1700 light years from Earth, in the constellation Cygnus. The
two stars whirl around each other every 11 hours. In 2013 and 2014, Molnar’s
team discovered
two things about KIC 9832227 that suggest an imminent explosion: the
orbital period is decreasing, and it’s doing so at an ever-faster rate.

This
is exactly what the contact binary that sparked the 2008 red nova did. The
orbital period shrank because the two stars circled each other faster as they
spiralled closer together. Unfortunately, other effects can mimic this decrease
in orbital period. For example, a third star can pull the binary toward us so
that its light takes less time to reach Earth, creating the illusion that the
two stars are circling each other faster. So additional observations were
needed to figure out what KIC 9832227 was likely to do.

In
late 2015, astronomers in Bulgaria observed the star with a 30-centimetre
telescope, and found that its period is still shrinking at an ever-faster clip.
“A stellar merger is a real possibility,” says Alexander
Kurtenkov of the University of Sofia. Molnar’s team finds this trend
persisting into 2016. “At this point, I think we have a serious candidate,” he
says.

His
latest observations, made with 40-centimetre telescopes in Michigan and New
Mexico, put the date of the potential explosion between 2021 and 2023. But he
cautions that another three years of observations are required before he can
rule out alternatives. By then, if the orbital period keeps shrinking faster
and faster, an impending explosion will be very likely. If it calms down, there
might be a different outcome.

KIC
9832227 is currently 12th magnitude – visible only through a telescope. But if
it brightens by 10 magnitudes, as the 2008 red nova did, it will be as bright
as the North Star and the brightest stars of the Big Dipper, and easily visible
to the naked eye.